hdu 2036 改革春风吹满地 任意多边形面积的求解

向量的叉乘求解任意多边形的面积，给出的点坐标要顺时针（结果为负）或逆时针（结果为正）

//任意多边形面积=向量的叉乘//#include <iostream>#include <cmath>using namespace std;typedef struct node{double x, y;}Tpoint;double polygonArea( Tpoint p[], int n ){double area = 0;int i, j;for( i=0; i<n; i++ ){j = (i+1)%n;area += p[i].x*p[j].y - p[j].x*p[i].y;}area /= 2.0;return fabs(area);}int main(){Tpoint p[105];int n;while(scanf("%d", &n) && n){int i;for( i=0; i < n; i++ )scanf("%lf%lf",&p[i].x, &p[i].y);printf("%.1lf\n", polygonArea(p, n));}return 0;}